1. Field of the Invention
The present invention relates to cooling and filtering devices for hot working fluids and, more particularly, to a counter-flow-type liquid -to-liquid heat transfer device adapted for the cooling and filtering of the hydraulic fluid which serves as the pressure medium in a hydraulic drive system.
2. Description of the Prior Art
Several devices which provide both a cooling and a filtering function for hot working fluids are known from the prior art. One such device, disclosed in the German Patent No. 28 34 399, is adapted for the cooling and filtering of the lubricating oil of internal combustion engines. In this device, the hot fluid first traverses a filter cartridge in a radial inward direction, whereupon it enters a heat transfer section of the housing which extends transversely away from the filter section.
Inside the heat transfer section, the hot filtered fluid flows around a central hollow heat transfer cylinder with a series of annular ribs on its circumference which transfer heat from the fluid to a cooling medium flowing on the inside of the heat transfer cylinder. Because of its angular outline, this device requires a considerable amount of space and its cooling unit is comparatively ineffective, having only a few annular ribs on a single short heat transfer element.
Another fluid cooling and filtering device is disclosed in the German Gebrauchsmuster (Utility Model) No. 76 15 571. This device, adapted for mounting inside the fluid reservoir of a machine, suspended from a cover on its top wall, has an upper filtering section and an axially aligned lower cooling section. The used hot fluid enters the device at its upper end, passing radially through a filter cartridge and from there downwardly into an annular coil chamber which is occupied by the double coil of a finned heat transfer pipe. The device may be arranged for either a vertically upward or a vertically downward flow of the filtered fluid in the coil chamber.
This known device has the shortcoming of requiring a considerable amount of space, while being limited in its mounting possibilities to the top wall of a fluid reservoir. The device has a comparatively complex structure, presenting access problems for cleaning and inspection purposes. The double-helix heat transfer coil requires a radially large coil chamber, where major portions of the flow space are not occupied by pipe fins and therefore can be bypassed by the fluid.
The use of helical fins on heat transfer pipe is likewise known from the prior art: See, for example, pages 10 and 13 of "Rippenrohre"--Catalog 830.03/January 1982 of R. & G. Schmole Metallwerke GmbH & Co. KG, 5750 Menden, Federal Republic of Germany. This catalog shows that the helical fin may be attached to the outer circumference of a pipe wall by soldering, by frictional engagement with an L-shaped inner edge of the fin, or by swaging attachment to a helical groove in the pipe.
Alternatively, the helical fin may be an integral part of the wall of the heat transfer pipe itself, being produced in a rolling and shaping operation in which the pipe wall is extruded radially outwardly between cooperating shaping tools, in a rolling operation which is similar to a thread rolling operation.
A disadvantage of the integral helical fin, apart from its restriction to pipe stock of highly malleable metal, such as copper and some of its alloys, resides in the fact that the rolling operation produces a fin which may have an uneven outer diameter and, consequently, may exhibit noticeable deviations from a true helix. A heat transfer coil employing this kind of pipe, therefore, requires a greater axial distance between coil turns and a coil chamber which is wide enough to accommodate the maximum fin diameter, with the result that space is wasted inside the coil chamber and it is easier for the fluid to bypass the fins of the heat transfer pipe.